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Development of a biogeochemical reaction model to simulate net methylmercury production in sediments
Stefanie HelmrichUniversity of California, Merced
Mercury Cycle in Reservoirs
Alpers C. N., Hunerlach M. P., May J. T. and Hothem R. L. (2005) Mercury Contamination from Historical Gold Mining in California. Publ. US Geol. Surv. 61, 7.
Conceptual Model for Hg Reaction Model
Model implemented in PHREEQC(Bessinger B. a., Vlassopoulos D., Serrano S. and O’Day P. a. (2012) Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury. Aquat. Geochemistry 18, 297–326.)
Methylation rate
Methylation rate formulation in experiments
𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 =𝑑𝑑 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+
𝑑𝑑𝑑𝑑= 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � 𝐶𝐶𝐻𝐻2+
𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = Methylation rate constant𝐶𝐶𝐻𝐻2+ = Concentration inorganic Hg𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+ = Concentration methylmercury
Methylation rate
Methylation rate formulation in experiments
𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 =𝑑𝑑 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+
𝑑𝑑𝑑𝑑= 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � 𝐶𝐶𝐻𝐻2+
Methylation rate formulation in model Coupled withIron reduction 𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑓𝑓𝐹𝐹𝑚𝑚𝐹𝐹𝐹𝐹𝐻𝐻 � 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � 𝐶𝐶𝐻𝐻2+
Sulfate reduction 𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑓𝑓𝑆𝑆𝐹𝐹𝑆 � 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � [𝐶𝐶𝐻𝐻2+]Methanogenesis 𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑓𝑓𝐶𝐶𝐻𝐻𝑆 � 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � [𝐶𝐶𝐻𝐻2+]
𝑓𝑓𝐹𝐹𝑚𝑚𝐹𝐹𝐹𝐹𝐻𝐻 ,𝑓𝑓𝑆𝑆𝐹𝐹𝑆, 𝑓𝑓𝐶𝐶𝐻𝐻𝑆 = Fraction of electrons consumed by reduction half-reaction𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = Methylation rate constant𝐶𝐶𝐻𝐻2+ = Concentration inorganic Hg𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+ = Concentration methylmercury
Thank you.
Use of isotope tracers to calulcate methylation rate constants
Methylmercury (MeHg) concentration over time
Methylation Rate Pseudo first-order reaction
𝑑𝑑 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+
𝑑𝑑𝑑𝑑= 𝑘𝑘𝑚𝑚 𝐶𝐶𝐻𝐻2+
Assumptions• No demethylation• No change in 𝐶𝐶𝐻𝐻2+
concentration
Hintelmann H., Keppel-Jones K. and Evans D. (2000) Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211.
Spike of isotopocially labeled 𝐶𝐶𝐻𝐻2+ and 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+
Comparison measured and simulated MeHg concentration
• Good fit during 24 hours• Assumptions not correct for longer incubationtime
Use of isotope tracers to calulcate methylation rate constants
Biogeochemical reaction model• PHREEQC (Version 3), U.S. Geolological Survey• Model capabilities:
• Heterogenneous and homogeneous equilibrium speciation• Kinetic-based mineral dissolution and precipitation• Kinetic-based biodegradation organic matter via redox reactions• Methylation and Demethylation of mercury
Evaluating Nutrient Fate and Redox Controls in Groundwater in Riparian Areas, Michelle M. Lorah (USGS MD-DE-DC Water Science Center, Baltimore, Maryland), Isabelle M. Cozzarelli and J. K. Böhlke (USGS National Research Program, Virginia, 03/30/2017
Kinetic rates for redox reactions according to: Canavan R. W. et. al. (2006) Organic matter mineralization in sediment of a coastal freshwater lake and response to salinization. Geochim. Cosmochim. Acta 70, 2836–2855.Van Cappellen P. and Wang Y. (1996) Cycling of iron and manganese in surface and sediments: a general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron, and manganese. Am. J. Sci. 296, 197 – 243.
Redox reactions
Evaluating Nutrient Fate and Redox Controls in Groundwater in Riparian Areas, Michelle M. Lorah (USGS MD-DE-DC Water Science Center, Baltimore, Maryland), Isabelle M. Cozzarelli and J. K. Böhlke (USGS National Research Program, Virginia, 03/30/2017
Kinetic rates for redox reactions according to: Canavan R. W. et. al. (2006) Organic matter mineralization in sediment of a coastal freshwater lake and response to salinization. Geochim. Cosmochim. Acta 70, 2836–2855.Van Cappellen P. and Wang Y. (1996) Cycling of iron and manganese in surface and sediments: a general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron, and manganese. Am. J. Sci. 296, 197 – 243.
Use of isotope tracers to calulcate methylation rate constants
Methylmercury (MeHg) concentration over time
Hintelmann H., Keppel-Jones K. and Evans D. (2000) Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211.
Spike of isotopocially labeled 𝐶𝐶𝐻𝐻2+ and 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+
Use of isotope tracers to calulcate methylation rate constants
Number of studies using isotope tracers to calculate methylation rate constants
36
Calculated methylation rate constant
(𝑑𝑑−1)
0.0006 – 1.5
Use of isotope tracers to calulcate methylation rate constants
Number of studies using isotope tracers to calculate methylation rate constants
36
Calculated methylation rate constant
(𝑑𝑑−1)
0.0006 – 1.5
Why such a variation in magnitude?Which order of magnitude is plausible?
Use of isotope tracers to calulcate methylation rate constants
Fraction of MeHg produced from isotopically labeled HgJonsson S., Skyllberg U., Nilsson M. B., WestlundP., Shchukarev A., Lundberg E. and Bjo E. (2012) Mercury Methylation Rates for Geochemically relevant Hg(II) species in Sediments. Environ. Sci. Technol. 46, 11653–11659.
0
0.05
0.1
0.15
0.2
0 5 10 15 20 25
MeH
g/TH
g fr
actio
n
Time in days
MeHg/THg fraction, first batch MeHg/THg fraction, second batch
0
0.005
0.01
0.015
0.02
0.025
0.03
0 2 4 6 8
MM
Hg (n
mol
/g)
Time in days
MeHg produced from isotopically labeled HgMartin-Doimeadios R., Tessier E., Amouroux D., Guyoneaud R., Duran R., Caumette P. and Donard O. F. X. (2004) Mercury methylation / demethylation and volatilization pathways in estuarine sediment slurries using species-specific enriched. Mar. Chem. 90, 107–123.
Influence incubation time used to caluclate rate constant
Start: Methylation >> Demethylation
After approximately 24 to 48 hours:Methylation = Demethylation
Use of isotope tracers to calulcate methylation rate constants
Influence incubation time used to caluclate rate constant
2 - 5 18 - 24 48 960.0
2.0
4.0
6.0
8.0
10.0
12.0
Harm
onic
mea
n fo
r max
km
, 1*
10^-
2 (d
-1)
Incubationtime in hours
Hintelmann H., Keppel-Jones K. and Evans D. (2000) Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211.
Reasonable order of magnitude:1 � 10−2𝑑𝑑−1
Preleminary Results
Influence of organic matter on methylation rate
Influence of organic matter on methylation rateSensitivity analysisHigh ROM Reactive Organic matter = 500 𝑚𝑚𝑚𝑚Low ROM Reactive Organic matter = 5 𝑚𝑚𝑚𝑚
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